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(CNN) — It was another December afternoon back in 2000, spent like hundreds of others combing the rocky hills of the Dikika region, when Ethiopian scientist Zeray Alemseged heard one of his assistants nearby calling him.

“He said ‘oh, doctor I see something there,’” recalls Alemseged, who’d been excavating the hot and dry landscape for over a year, helped only by a small inexperienced crew of locals. “And I went there and I see the cheek bone part … sticking out of the rock. I turned it upside down and my jaw literally dropped.”

Instantly, Alemseged realized this was an extraordinary discovery that could make scientific history.

“Right away I could tell this is a child of a human ancestor,” says the paleoanthropologist. “You have this child in a block of sandstone, with the baby teeth still visible, very vertical forehead, small canine,” he adds. “But it’s so rare and so unbelievable that I just couldn’t accept that was the case.”